In organic photoconductor formulations, the wear performance of the photoconductor is a key factor in determining the useful life of the print cartridge. Photoconductor wear comes from many sources in an electrophotographic engine, including photoconductor contact with the cleaner blade, charge roll, and other spacers or seals that may be a part of the print engine. Wear in the transport layer may cause an overall reduction in thickness of the coating, or it may cause a localized defect in the coating at a specific point of contact with another engine component (e.g., a groove cut in the transport coating at the edge of the charge roll). If the coating becomes thinner, its capacitance will increase and several electrophotographic processes can be affected. For example, the transfer of toner from the photoconductor to the paper may degrade, or if the coating wears away completely, toner may develop to the worn spot and create a print defect. Excessive current also may flow in that spot, and components such as the charge roll may fail prematurely from the high current flow. At that point, either the photoconductor or the entire print cartridge must be replaced to obtain original print quality.
The print performance of the photoconductor can change over its useable life as a result of several factors. One of these factors is the discharge voltage effected by the laser print head. The discharge voltage can become higher or lower over the life of the photoconductor, depending upon the materials used in the formulation of the photoconductor. In discharge area development, the discharge voltage decreases (lower negative voltage) and the print becomes darker. In charge area development, the charge voltage increases (higher positive voltage) and the print becomes darker. This is particularly noticeable when the print is in the form of graphics, illustrations, and pictures which require different shades of blacks or greys. The darkening of the print results in loss of the resolution between the different greyscales, and thus print quality is lost.
The wear performance of the photoconductor depends on the mechanical properties of the charge transport layer. The charge transport layer is formulated from two major components: a polymeric binder resin and a charge transport material. The binder resin is chosen to impart the physical durability necessary for an acceptable useful life under the service conditions encountered in copiers and printers. Typically, the polymeric binder resin is doped with the charge transport material, which often acts as a plasticizer, thereby compromising the mechanical properties of the binder.
Aromatic polycarbonates are one class of resins which can be used as binders in the charge transport layer. Two examples of polycarbonate resins that may be used are bisphenol-Z polycarbonate and bisphenol-A polycarbonate. The doped bisphenol-Z polycarbonates are inherently more wear resistant than the doped bisphenol-A polycarbonates. It is nevertheless desirable to use a, bisphenol-A polycarbonate as a binder resin because it is readily commercially available and relatively inexpensive. Until the present invention, however, no one has been able to provide a charge transport formulation which is suitably resistant to surface scratching and wear during the copy or print process and which retains good mechanical and electrical properties as compared with similar charge transport formulations without any wear-improving additive. Further, no one has been able to provide such a charge transport formulation which uses a relatively inexpensive binder resin such as bisphenol-A polycarbonate.
A number of different approaches have been taken to reduce wear in OPC charge transport formulations. One approach is to coat a third layer, typically called an overcoat layer, on top of the charge generation and charge transport layers. The overcoat is typically a very thin (1-2 microns) polymeric layer which contains little or no charge transport dopant and which possesses improved mechanical properties relative to the charge transport layer. There are, however, several drawbacks to using the overcoating approach to improve wear. First, an additional step in the organic photoconductor coating process adds significant cost to the finished photoconductor. Also, it is difficult to coat an additional layer on top of the charge transport layer without partially dissolving it; this difficulty can be overcome by the proper choice of solvents for the overcoating material, or by utilizing a different coating method (e.g., spray coating) for the overcoat. Nevertheless, an additional coating step (and possibly additional coating equipment) still will add to the expense of the finished organic photoconductor. Finally, it is difficult to add an insulative coating to the photoconductor surface without changing its fundamental electrostatic performance. Typically, addition of an overcoat layer causes a loss of residual voltage and/or an overall reduction in sensitivity. The addition of an overcoat also may change the performance of the photoconductive drum, either electrically or mechanically, at extreme environmental conditions (i.e., high or low ranges of temperature or humidity).
Another approach to reducing charge transport layer wear is to add materials directly to the transport formulation that will modify the mechanical properties of the coating. This provides the advantage of not requiring an additional step in the drum coating process. A number of different materials have been used in this manner, including fluoropolymer particles, inorganic oxides, and various types of silicone oils. The potential disadvantage to this method is that the fundamental properties of the charge transport layer may be changed by the presence of the additive. Similar to the overcoating layers, whatever method is used to improve wear properties should have little or no effect on the electrostatic properties of the photoconductive drum. A change affecting the hardness of the formulation may also affect the attraction of toner to the drum, or may lead to incomplete cleaning of the drum (fused-on-toner). If either the electrical or mechanical properties of the drum are changed, it will likely be manifested in print quality changes that can be directly related to the presence of the additive.